Most of the devices developed to date for microwave power amplification, can be broadly classified as being either "solid state" or "thermionic" in nature. The solid state class of these devices generally utilize a semiconductor material such as silicon or gallium arsenide, and are dominated by transistor structures such as the metal-semiconductor field effect transistor (MESFET), or the heterojunction bipolar transistor (HBT). These devices have excellent frequency response capability, with unity gain frequencies up to several hundred gigahertz, along with good input/output isolation and good stability. However, since these transistors must operate at bias levels of typically 8 to 12 volts, their power densities are limited to about 100 watts per square centimeter and it becomes impractical to design discrete devices with power levels exceeding a few watts per device.
Thermionic devices, or vacuum tubes, have traditionally served as the "workhorse" for high power amplification at microwave frequencies. These devices, such as the traveling wave tube (TWT) and the crossed field amplifier (CFA) are capable of providing output powers of from a few watts to megawatts. However, these tubes generally operate at difficult to handle voltage levels of several thousand volts, and are generally large and bulky due to the electron beam focussing and attendant magnet structure required for operation. In addition, these tubes generally employ complicated, precision three-dimensional circuit structures that are expensive to fabricate and are generally not amenable to mass production techniques.
The patent art discloses several examples of inventions of general background interest with respect to the present invention. Included in these examples is the U.S. Pat. No. 4,721,885 of Ivor Brodie which is concerned with very High Speed Integrated Microelectronic Tubes; the U.S. Pat. No. 4,780,684 of H. G. Kosmahl which is concerned with a Microwave Integrated Distributed Amplifier with Field Emission Triodes, the U.S. Pat. No. 4,901,028 of H. F. Gray et al which is concerned with Field Emitter Array Integrated Distributed Amplifiers; and the U.S. Pat. No. 4,987,377 also of H. F. Gray et al which is additionally concerned with distributed amplifiers and is a continuation of the '028 patent.
Although each of these patents relates to field emission electronic devices none of these patents teaches the combination of a field emission device with a solid state transistor device and the number of significant improvements which can be achieved therewith.